1. Field of the Invention
The present invention relates to a tappet made of light metal such as aluminum alloy having a hard cam receiving plate secured to a top surface of the tappet, and also relates to a method of manufacturing the tappet.
2. Description of the Prior Art
Recently, a tappet used for a direct-acting type valve operating mechanism of an internal combustion engine tends to be made of aluminum alloy for reducing inertial mass of a valve operating system.
In such a tappet made of aluminum alloy, a shim made of hard metal having abrasion resistance is fitted to a recess formed on a top surface of the tappet for preventing abrasion of the top surface on which a cam slides.
However, if such a shim is used, management of the parts is troublesome, and not only the assembling line of the engine is complicated, but also effective surface area of a sliding portion of the top surface of the tappet with the cam is reduced. Therefore, if a lift amount of an engine valve is to be increased, it is necessary to increase an outer diameter of a tappet body, and to mount a cam having a large diameter.
If doing so, a cylinder head is also enlarged, and a flexibility in engine design is limited.
To solve such a problem, there is proposed a tappet in which a cam receiving plate made of hard metal is integrally secured to the entire top surface of a tappet body made of aluminum alloy.
The above-described cam receiving plate is secured on the top surface of the tappet body by electric resistance welding, brazing or the like. However, in order to couple different materials having largely different melting points, it is necessary to strictly manage the heating temperatures of both the material, such operation is troublesome, and coupling strength is prone to be deviated.
Further, aluminum alloy has a great activity, and metal oxide is prone to be formed on a surface thereof. Therefore, if both the materials are simply abutted against each other, there is a problem that impurity such as the metal oxide is mixed into the joint boundary, and a sufficient coupling strength can not be obtained.